This invention concerns a coil assembly incorporated into apparatus for measuring the density of a fluid.
In U.S. Pat. No. 3,763,692 there is disclosed an apparatus for measuring the density of a fluid in which the fluid is caused to pass through a sensing tube, the sensing tube is excited by a drive coil assembly to vibrate at a resonant frequency, and a signal, whose frequency depends upon the density of the fluid passing through the vibrating sensing tube, is produced in a detecting coil assembly.
Each of the said coil assemblies of U.S. Pat. No. 3,763,692 comprises a magnetic housing which is disposed adjacent to the sensing tube and is open on the side facing the latter, the housing containing a drive or detecting coil. The housing has a peripheral wall and a central pole piece which is coaxial with the peripheral wall and extends through said coil.
In each of the coil assemblies of U.S. Pat. No. 3,763,692 therefore, the magnetic flux between the peripheral wall and the pole piece passes outwardly of the housing through the said open side thereof and thus contacts the sensing tube over a relatively large area. In consequence, magnetic cross-coupling can occur between the two coil assemblies and this can give rise to errors in density measurement, e.g. by causing "mode jumping", i.e. by causing the mode of vibration of the sensing tube to alter.
This problem has been particularly severe in the case of such density measuring apparatus intended to be operated at high temperatures because in that case the coil has to be made of a specifically insulated wire, e.g. a ceramic insulated nickel clad copper wire, and this means that the size of the coil -- and hence the size of the housing has to be greatly increased. This in turn increases the size of the said open side of the housing through which the magnetic flux passes and therefore increases the area of the sensing tube against which the said flux impinges with consequentially increased risk of magnetic cross-coupling between the two coil assemblies.
Accordingly it is an object of the invention to provide a coil assembly which, when used in an apparatus for measuring the density of a fluid, will obviate the risk of such cross-coupling.
Another object of the invention is to provide such a coil assembly suitable for use at high temperatures.